The biochemical aberrations in extrapyramidal dysfunctions were investigated. Monkeys with ventromedial tegmental lesions were used as test models to study the anti-tremor efficacy of various drugs. Dopamine receptor stimulating agents were found to relieve and to induce involuntary movements in experimental monkeys. These results indicate that tremor and involuntary movements are associated with a common mechanism and that the activity of the dopamine receptors is involved in the regulation of these dysfunctions. The clinical antiparkinsonian efficacy of various agents that stimulate dopamine receptors is now under investigation. The inhibition of synaptosomal hydroxylase activity by various dopamine receptor stimulating agents and the reversal of the inhibition by dopamine receptor blocking agents is now being tested as a biochemical model for screening antiparkinsonian drugs.